An optical circulator is a non-reciprocal optical device having important roles in an optical communication system, optical measurement or the like. The optical circulator has at least three ports. For example, in case of an optical circulator having three ports indicated by reference numerals 1, 2, and 3, in a forward direction of 1->2, 2->3, 3->1, light is propagated with low loss, and in a backward direction of 3->2, 2->1, 1->3, light is propagated with high loss.
However, since the optical circulator has a structure where light is propagated from one port to other opposite ports in the propagation direction, there is a problem that the number of optical elements is increased, so that the optical circulator has a large size. In addition, if the number of ports is increased, the number of optical elements is further increased, thus the optical elements has a larger size. Therefore, there is a problem that it is difficult to implement the optical circulator having multiple ports. For this reason, there has been proposed a reflective optical circulator having a reflector for forming a reciprocating optical path (for example, refer to Patent Documents 1 and 2). In the reflective optical circulator, the number of optical elements is not increased even in case of having multiple ports, and its size is small in comparison with a conventional structure.
Patent Document 1: Japanese Patent Application Publication No. 2000-231080 (Pages 2 to 5, FIGS. 1 to 6
Patent Document 2: Japanese Patent Application Publication (Japanese Publication of PCT Application) No. 2002-528765 (Pages 38 to 39, FIG. 12
As shown in FIG. 16, in an optical circulator 100 disclosed in Patent Document 1, four array-type optical fibers 101 are used in an light incidence/emission port (light incidence/emission unit), and a birefringent element 103, a first phase element 104 including two half-wave elements, a polarization plane rotating element 105, and a composite birefringent element 106 including two birefringent elements are disposed between the array-type optical fibers 101 and a lens 102. In addition, a second phase element 107, and a reflector 108 are disposed behind the lens 102.
As shown in FIG. 17, in an optical circulator 109 disclosed in Patent Document 2, a non-linearly-arrayed optical fiber bundle 110 is used in an light incidence/emission port (light incidence/emission unit), and two birefringent elements 111a and 111b, two Faraday rotators 112 and 113 (or a polarization plane rotating element 114), and a reflecting prism 115 as a reflector are disposed. The rotation directions of the two Faraday rotators 112 and 113 are adapted to be opposite to each other.
However, as shown in FIG. 18, the four optical fibers 101 constituting light incidence/emission ports of the optical circulator 100 are disposed in a linear array constitution as seen in the z-axis direction of FIG. 16. Therefore, central axes of the optical fibers P1 to P4 cannot be disposed at equivalent distance from a central point C of diagonal lines connecting the central axes of the optical fibers P1 to P4, and outer optical fibers P3 and P4 are disposed at longer distance from the central point C. As a result, a difference in optical path length between the inner reciprocating optical path (for example, P1->P2) and the outer reciprocating optical path (for example, P3->P4) occurs, and thus, non-uniformity of insertion losses of reciprocating optical paths occurs. Accordingly, in the conventional optical circulator 100, it is difficult to stabilize the insertion losses.